Can only unvote certain stories?

[swiper]

I just noticed something weird with my pligg site. If I go to the Voted tab of my profile it shows all the stories I voted for. About half of them I can Unvote by clicking the Unvote link on the button, but about half of them don't have this and simply have the grayed out Voted text. Is there some other built-in criteria that determines what you can unvote for, such as you can't unvote after a certain time has passed or something??

[tbones]

Could it be that you get a dynamic IP by your provider?

Unvote limited by IP - wise?